Plain and simple, the Church has not abolished the law requiring Catholics to participate in the celebration of Mass on Sundays and holy days of obligation.
Canon 1247 reads, "On Sunday and other holy days of obligation the faithful are obliged to participate in the Mass...."
Note, there is a precept to participate in Mass on Sundays and holy days of obligation and it binds gravely. At the same time there can be serious reasons that excuse a person from observing the law.
Manuals of theology published before the present Code of Canon Law spoke of moderately grave reasons that would excuse. Besides illness, distance from the church, police duty, the need to shut down mills that run around the clock, the grave displeasure of a spouse or parents, the demand of an employer, fire and flood emergencies, care of the ill and being on a journey were listed as examples of such moderately grave reasons.
Some moralists believe to miss Sunday Mass on one or the other time without an excusing cause would not be a serious sin—unless done with contempt of the law. It may be that some people translate this to say it’s no longer a sin to miss Mass on Sunday, but that is not what these moralists are saying.